Beryl's remnants will be lifting through the Great Lakes Region today. This will lift a warm front through New England during the day. This warm front will likely slow down or even stall out across central Vermont, New Hampshire and Maine due to an approaching cold front from Canada. This front will provide the focus for a swath of heavy rainfall, where multiple inches of rain will be possible. Areas north and south of this front will see lesser rainfall. Areas south may also see some strong to severe storms in the warm sector.
ALERTS
TIMING
Showers will break out in earnest Wednesday afternoon across Vermont. The showers will gradually fill in, becoming heavier as they do so. The heavier showers will steadily push eastward, arriving in Maine by early Wednesday evening. Strong to severe thunderstorms will be possible along with the afternoon and evening rainfall, which would not only raise the rainfall rate, but introduce strong winds as well. Strong thunderstorms will be most likely across southern Vermont, southwest New Hampshire and western Massachusetts.
Potential weather around mid-afternoon (1st image) and this evening (2nd image):
Bands of steady to heavy rain will continue through the night for the northern tier of New England as the warm front continues to slowly lift through the region. A triple point low will likely form in the evening or overnight hours, adding additional forcing for rainfall. The heaviest of the rain will come in the overnight hours for most. Areas farther south will see only occasional showers.
Potential weather around midnight tonight:
This will not be a long duration event as the storm shuts down in the morning for Vermont and New Hampshire. An overall drying trend will occur as Thursday goes on with eastern Maine seeing the most widespread rain in the early afternoon before beginning to taper off.
Potential weather early Thursday afternoon:
RAINFALL/FLOODING
When it comes down to it, the placement of the warm front is the key to understanding where the heaviest strip of rain will fall. Wherever the warm front slows down this afternoon and evening will be the area that sees 2+ inches of rain. The question is how far north this front gets before slowing down. A cold front in Canada will help slow the warm front down. Some models have continued to push the heaviest rain farther north into northernmost Vermont and New Hampshire or even into Canada.
Since warm fronts can move more slowly through New England than some models think, the band of heaviest rain will likely occur in New England across the northern tier, but there is still a spread on exactly where this will set up. This band will likely see rainfall of 2-3+ inches. Any thunderstorm development will enhance total rainfall. There will likely be a pretty sharp cutoff heading south in New England.
The overall setup is favorable for bouts of heavy rain and torrential downpours. New England is already entrenched within very moist air as an expansive high pressure over the Atlantic is pumping very humid air into the region on a persistent southwest flow. The remnants of a tropical system will help inject more moisture into the atmosphere as it approaches. A frontal boundary has been stalled over New England since this past weekend, which will provide the forcing for rainfall.
With an extremely high moisture-content in the atmosphere (precipitable water values will be in the 2 to 2.5 inch range, which indicates the potential for very heavy rainfall rates), torrential downpours will be produced. Rainfall rates in excess of 2 inches an hour will be possible at times (it won’t rain at 2 inches an hour for a full hour). Northern Vermont has been the wettest over the last month, seeing above average precipitation in June.
The current threshold for flash flooding to occur ranges from 1.5 to 2.5 inches in three hours for much of northern New England. The six hour threshold is around 2-3 inches. Rainfall rates are poised to exceed these thresholds in some areas, especially where thunderstorms set up within the line of heavier rain.
The Weather Prediction Center has placed northern and central Vermont, the White Mountains and the New Hampshire Lakes Region to the "moderate" risk (level 3 of 4) for flash flooding. This indicates the possibility for numerous flash floods with some serious flash floods possible. While some river flooding is possible, this will not be a major river flooding event.
Scattered afternoon thunderstorms should be progressive enough to limit flash flooding for southern and central New England, but any storm will be able to produce torrential rainfall. This is why a low flood risk extends south into southern New England. Storm coverage over southern New England looks to be very spotty at best.
SEVERE STORMS
There will be the potential for severe thunderstorms this afternoon and evening. This threat will be greatest to the south of the warm front, well into the warm sector. The four ingredients for severe weather will be in place. Shear will be strong, lift will be provided by the warm front and instability and moisture will be present within the warm sector. The severe threat will be greatest in the afternoon with the threat diminishing after sunset, as per usual.
The greatest risk with severe storms will be strong wind gusts. With a triple point low forming and strong wind shear, there is a threat for rotating supercells and tornadoes. This is highlighted by the Storm Prediction Center having the area in yellow above in the 5% risk of tornadoes. This basically means there is a 5% chance of any one spot seeing a tornado. The SPC has a strip of New York in the 10% risk. Large hail isn't impossible, but it's unlikely to pose much of a threat.
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